Ultra high frequency amplifier



July 28, 1942. 1.. J. WOLF ULTRA HIGH FREQUENCY AMPLIFIER Filed Jill 28, 1939 E 25 :QEEEx INVENTOR.

ATTORNEY.

Patented July 28, 1942 UNITED STATES PATENT OFFICE 2,291,428 ULTRA men FREQUENCY AMPLIFIER.

Lester J. Wolf, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 28, 1939, Serial No. 286,977

-5 Claims.- (Cl. 1'f79171) impedance variations of the said input upon the output of the preceding stage.

Another object is to provide an efilcient radio frequency amplifier circuitconsisting of a pair of cascaded push-pull stages coupled together by a circuit insuring a desired amount of radio frequency drive impressed upon the last of said stages, and satisfactory grid regulation therefor.

A feature of the present invention involves the use of a coupling circuit between the output of one radio frequency amplifier stage and the input of a. succeeding radio frequency amplifier stage, employing a pair of nichrome resistors in the form of a loop for inductively coupling said stages together.

A more detailed description of the invention follows, in conjunction with a drawing whose single figure shows, schematically, an amplifier circuit embodying the principles of the invention.

In the drawing, an amplifier stage comprising vacuum tubes I3 and I4 connected in push-pull relation is arranged to drive a second amplifier stage I, 2, also in push-pull relation. Both of these stages are provided with cross-neutralization schemes for eliminating capacity coupling between the associated input and output circuits, so as to stabilize the amplifier and prevent selfoscillation. The tuned input circuit 26 of the first stage I3, I4 is coupled to a preceding oscillator, or other suitable source of high frequency energy, not shown, while the output circuit 2I, 22 of the second stage I, 2 is coupled to a transmission line extending to an antenna. The input circuit of stage l3, I4 is provided with resis'tors 20, which enable the adjustment of individual bias for the grids of-tubes I3 and I4- 2 comprises an inductance 2I shunted by a variable condenser 22. Inductance 2| is in the form of a parallel conductor loop which is electrically tending from semi-cylindrical plate t to a distenna system, not shown. Condenser 25 serves to tune out the reactance of coil 2 The active parts of the filaments of vacuum tubes 5 and 2 are maintained at ground or zero radio frequency potential by means of connections which are one-half wavelength long and tuned or maintained at this length by slider 36. The mid point of the filament of each tube is thus efiectively grounded for radio frequency potentials and the filament leads including the lengths within the vacuum tubes are tuned out.

The cross-neutralization scheme, used in pushpull amplifier stage i, 2 departs somewhat from the conventional neutralizing circuit shown used in connection with amplifier stage I3, M, in that capacitors are provided for compensating forthe reactance of the leads in the neutralizing circuit.

" The main neutralizing capacitors are made by semi-cylindrical outer armatures or plates 3 and i placed around the anode jackets oftubes i and 2, respectively. formed by plate 3 and the anode jacket of tube ihas a value equal to the grid-anode capacity of tube 2. Similarly, the neutralizing capacitor formed by plate a and the anode jacket of tube 2 has a value equal to the grid-anode capacity of tube I. Capacitor 5 compensates for the reactance of the lead extending from semi-cylindrical plate 3 to a distance substantially half way betweencapacitors 5 and 6. Similarly, capacitor 9 compensates for the reactance of the lead extance half way between capacitors 9 and I. In the same way, capacitor I in series with the grid of vacuum tube I compensates for the reactance of the lead extending from the grid of vacuum tube I to a distance half way between capacitors I and 9. Also, capacitor 6 in series with the grid of vacuum tube 2 compensates for the reactance of the lead extending from the grid of vacuum tube 2 to a distance half way between capacitors 6 and 5;

The driving stage I3, I4 is not provided with compensating capacitors-5, Ii, 1 and 9, because the leads going to'the neutralizing elements of tubes I3 and I4 do not have the reactances that exist in. the leads in circuit with tubes I and 2.

I In practice, vacuum tubes, I3 and N are small,

one-quarter wavelength long from the center of l the shorting bar to the anodes of the tubes I and 2. Coupled to the inductance 2I is a coil 2 in series with a condenser 25, both connected in perhaps seven or eight inches over-all length, whereas the length of vacuum tubes l and 2 is perhaps some four feet, more or less.

The output circuit for the first stage comprises a parallel conductor loop l5 having a variable trimmer condenser II in shunt therewith for turn to a coaxial cable 23 extending to an antuning the loop. Thisloop- Ii. which develops a The neutralizing capacitor high ciru'lating current for radio frequency, is electrically one-quarter of a wavelength long, as measured from each anode of tubes I3, I4 to the midpoint of the shorting bar across the legs of the loop I5. The resistor IS in the output circuit serves to reduce parasitic currents.

- For coupling the output circuit I5 to the grids of the succeeding amplifier stage I, 2, there is provided a coupling circuit II which can be consid: ered as consisting of two parts separated by condensers I6, I6; first,'a loop II to the-right ofthe condensers .I 6, I8 which is electrically one-quarter I wavelength long and is inductively coupled to the output loop l5, and secondly, a section to the left of condensers I6, I6 which is electrically one-half wavelength long.

The electrical half wavelength section between "condensers I6, I6 and the grids of tubes I, 2 includes a pair of nichrome resistors 8 and III which are characterized by an extremely high resistance-to-inductance ratio and are non-reactively wound to further diminish the reactive component. The resistors 8 and III serve the purpose of minimizing the effects of wide impedance variations of the grids of tubes I and-2 upon the transmission line H. In practice, the modulation impressed upon the grids of tubes I and 2 is effected by varying the bias voltage on these tubes, but the ratio between the peak-topeak swing of that bias voltage and the peak value of the radio frequency voltage must be essentially unity. In the presence of the wide variations in impedance of the grids of tubes I and 2, and in the absence of the nichrome, the value of the radio frequency voltage would vary over limits which would not produce satisfactory modulation characteristics. Introduction of the nichrome .spirals 8 and I II reduces the impedance variations by coupling circuit or transmission line I I to the pointwhere the circuit is then practical. Ordinarily, the impedance variations across the grids of vacuum tubes I and 2 vary from perhaps 100 ohms'up to several thousand ohms. The insertionof the nichrome resistors 8, Sin the manner shown in the drawing prevents these large variations in impedance from being impressed upon the transmission line so that in practice the output of the vacuum tubes I3 and I4, in relation to the input of tubes I and 2, is

substantially constant. Putting it another way,

we can say the nichrome resistors 8 and I0 stabilize the impedance of the output or driving amplifier stage l3, I4.

Air blowers 3|, 3| serve to dissipate the heat developed in the nichrome resistors. I

The loop II includes a pair of nichrome resistors l2, I2 which are disposed in loose inductive coupling relation to the shorting bar across loop I5. The junction point of the nichrome resistors I2, I 2 is shown connected to the mid point of loop I5,although'if desired this connection may be eliminated and the junction point grounded. The loop including resistors I2, I2 diminishes the voltage appearing at the condensers I8, I6 and therefore diminishes the voltage appearing at the grids of tubes I and 2, while at the same time the nichrome resistors restrict the impedance variations in the transmission line II and provide improved control of tubes I3; and I4. Due to the resonant rise in current in the quarter wavelength loop II, by virtue'of its length, there is developed a high potential between capacitors I6,

IE on the'condenser plates'nearest the nichrome resistors I2, I2, at which location the i pedance of the circuit is very high. The condensers I6 are placed at a high radio frequency voltage, and

a low radio frequency current value because it is amount of radio frequency power through these,

condensers. Consequently these condensers are located in the circuit where the radio frequency voltage is very high and the radio frequency current is very low. Therefore, condensers having a very low capacity may be used without appreciable radio frequency voltage drop.

It should be noted that capacitors I and 6 serve to block the passage of direct current through the electrical half wavelength-section of the interstage coupling circuit. Modulating voltage is introduced at the grids of themddulating amplifier I, 2 through chokes I8 connected to lead 28 which extends to a modulator device not shown.

What is claimed is:

- 1. In an amplifier system having first and second stages of discharge tubes arranged for pushpull operation, an output circuit for the first stage and an input circuit for .the second stage, the

electrical lengths of said output and input circuits same so as to be disposed in capacitively reactive relation to theelectrodes within said tubes for neutralizing the inter-electrode capacitances of said second stage tubes.

2. An amplifier system for ultra high frequencies comprising an amplifier stage having a pairof vacuum tubes each-including a grid, a

cathode and an anode, an input circuit between said grids for applying ultra high frequency potentials thereto, a connection between said cathodes, and an output circuit comprising an electrically conducting loop of wire with a condenser in shunt th the legs of said loop coupled between said anodes, said loop having 'a length which is electrically one-quarter of a wavelength long at the operatingfrequency as measured from each anode to the electrical center of said loop, a succeeding .amplifier stage also having a pair of vacuum tubes each including a grid, 8. coupling circuit between the grids of said last stage and said output circuit for applying potentials to said grids which are out-of-phase relative to each other, said coupling circuit having a pair of nichrome resistors connected together, each of which is inductively coupled to a portion of said output circuit, a condenser in each side of said coupling circuit, the distance between each con-- denser and the junction point of said resistors being electrically one-quarter of. a wavelength at the operating frequency, said coupling circuit being electrically three-quarters of a wavelength long at the operating. frequency as measuredfrom each grid of thelast stage to the junction point of said nichrome resistors.

3. An amplifier system in accordance with claim 2, including a non-inductively wound output circuit, a condenser nichrome resistor located in said coupling circuit between each condenser and a grid of one tube of said succeeding amplifier stage.

4. An amplifier system for ultra high frequencies comprising an amplifier stage having a pair of vacuum tubes each including a grid, a cathode and an anode, an input circuit between said grids for applying ultra high frequency potentials thereto, a connection between said cathodes, and an output circuit comprising an electrically conducting loop of wire with a condenser in shunt to the legs of said loop coupled between said anodes, said loop having a length which is electrically one-quarter of a wavelength long at the operating frequency as measured from each anode to the electrical center of said loop, a succeeding amplifier stage also having a pair of vacuum tubes each including a grid, a coupling circuit between the grids of said last stage and said output circuit for applying potentials to said grids which are out-of-phase relative to each other, said coupling circuit having a pair. of nichrome resistors connected together, each of which is inductively coupled to a portion of said Y in each side of said coupling circuit, the distance between each condenser and the junction point of said resistors being electrically one-quarter of a wavelength at the operating frequency, said coupling circuit being electrically three-quarters of a wavelength long at the operating frequency as measured from each grid of the last stage to the junction point of said nichrome resistors, a nichrome resistor wound to reduce the reactive component located betwen each condenser and a grid of one of said tubes of said last amplifier stage, choke coil means connected across the grids of said last stage, and a connection to the center of said choke coil means for applying modulating potentials to the grids of said last stage.

5. A coupling circuit between a source of high frequency energy and a balanced circuit, comprising an electrically conducting loop having a pair of nichrome resistors directly connected together, each of which is inductively coupled to said source of high frequency energy. a condenser in each leg of said loop located between said resistors and said balanced circuit, said loop having a length which is electrically threequarters of a wavelength long as measured from said balanced circuit to the junction point of said resistors, the distance between said condensers and said junction point being electrically onequarter of awavelength at tha operating frequency, and cooling means of the air-blast type for dissipating heat which certain of said resistors radiate.

LESTER J. WOLF. 

